main: Also support CLI extensions in /usr/libexec/libostree/ext

In fixing https://github.com/coreos/rpm-ostree/pull/3323
I felt that it was a bit ugly we're installing `/usr/bin/ostree-container`.

It's kind of an implementation detail.  We want users to use
`ostree container`.

Let's support values outside of $PATH too.

For example, this also ensures that TAB completion for `ost` expands
to `ostree ` with a space.

src/libostree/ostree-1.pc.in

@@ -3,6 +3,7 @@ exec_prefix=@exec_prefix@
 libdir=@libdir@
 includedir=@includedir@
 features=@OSTREE_FEATURES@
+cliextdir=@pkglibexecdir@/ext
 
 Name: OSTree
 Description: Git for operating system binaries

src/ostree/ot-main.c

@@ -41,6 +41,10 @@ static gboolean opt_verbose;
 static gboolean opt_version;
 static gboolean opt_print_current_dir;
 
+// TODO: make this public?  But no one sane wants to use our C headers
+// to find where to put files.  Maybe we can make it printed by the CLI?
+#define _OSTREE_EXT_DIR PKGLIBEXECDIR "/ext"
+
 static GOptionEntry global_entries[] = {
   { "verbose", 'v', 0, G_OPTION_ARG_NONE, &opt_verbose, "Print debug information during command processing", NULL },
   { "version", 0, 0, G_OPTION_ARG_NONE, &opt_version, "Print version information and exit", NULL },
@@ -188,7 +192,7 @@ ostree_command_lookup_external (int argc,
 
   // Find the first verb (ignoring all earlier flags), then
   // check if it is a known native command. Otherwise, try to look it
-  // up in $PATH.
+  // up in /usr/lib/ostree/ostree-$cmd or $PATH.
   // We ignore argv[0] here, the ostree binary itself is not multicall.
   for (guint arg_index = 1; arg_index < argc; arg_index++)
     {
@@ -204,10 +208,18 @@ ostree_command_lookup_external (int argc,
             return NULL;
         }
 
+
       g_autofree gchar *ext_command = g_strdup_printf ("ostree-%s", current_arg);
+
+      /* First, search in our libdir /usr/lib/ostree/ostree-$cmd */
+      g_autofree char *ext_lib = g_strconcat (_OSTREE_EXT_DIR, "/", ext_command, NULL);
+      struct stat stbuf;
+      if (stat (ext_lib, &stbuf) == 0)
+        return g_steal_pointer (&ext_lib);
+
+      /* Otherwise, look in $PATH */
       if (g_find_program_in_path (ext_command) == NULL)
           return NULL;
-
       return g_steal_pointer (&ext_command);
     }